a) Field of the Invention
The present invention relates to a process for continuously producing a high-strength and high-modulus polyolefin material by continuously compression-forming a polyolefin powder at a temperature lower than the melting point of the powder and then rolling and stretching the polyolefin thus compression-formed.
b) Description of the Related Art
So-called ultra-high-molecular-weight polyolefins having a significantly high molecular weight are used in a variety of fields as engineering plastics featuring inter alia excellent impact resistance and abrasion resistance and self-lubricating property. Since these ultra-high-molecular-weight polyolefins have a far higher molecular weight compared to general-purpose polyolefins, they are expected to provide molded or otherwise formed (hereinafter collectively called "molded") products having high strength and high modulus provided that they can be oriented to a high degree. A variety of investigations have therefore been conducted with a view toward achieving their high orientation.
Such ultra-high-molecular-weight polyolefins however have a higher melt viscosity compared to general-purpose polyolefins, so that under the circumstances, they show extremely poor formability whichever conventional forming method is applied, and their high orientation by stretching is not feasible.
Paul Smith and Peter J. Lemstra proposed a process for producing a high-strength and high-modulus polyolefin material by stretching a gel, which had been obtained from a decalin solution (dope) of an ultra-high-molecular-weight polyolefin, at a high draw ratio (Japanese Patent Laid-Open No. 15408/1981). The process is however practiced only at an extremely low polymer concentration in the dope, as low as 3 wt.% for polyolefins having a weight average molecular weight of 1,500,000 and 1 wt.% for those having a weight average molecular weight of 4,000,000. This process is however very disadvantageous from the economical viewpoint because its practice requires the use of a solvent in a large volume and tremendous difficulties are encountered upon preparation and handling of a high-viscosity solution.
Various proposals have also been made on processes for stretching and orienting single-crystal mats of ultra-high-molecular polyolefins to a high degree [U.S. Pat. No. 4,545,950, Japanese Patent Laid-Open Nos. 15120/1985 and 97836/1985, Preprint of The Society of Polymer Science, Japan, 34(4), 873 (1985)].
In these processes, solid-phase extrusion, stretching or the like is however conducted using a single-crystal mat which has been obtained in advance by converting an ultra-high-molecular-weight polyolefin into a dilute solution of a solvent such as xylene, decalin or kerosine and then subjecting the solution to cooling or isothermal crystallization. Therefore, the problem that a great deal of solvent is indispensable upon production of a single-crystal mat has not been solved yet by these processes either.
With a view toward solving the above-describe problems, the present inventors proposed a process for the production of a high-strength and high-modulus polyolefin material, which comprises compression-molding a polyolefin powder of an ultra high molecular weight at a temperature lower than the melting point of the powder without dissolution or melting and then rolling and stretching the polyolefin thus compression-molded (EP 253513).
However, the compression-molding step in the process disclosed therein is of the batch system that a polyolefin powder is held between heated, upper and lower pressing platens and then heated and pressed there for a predetermined time to obtain a pre-molded sheet. The above process thus involves problems such that it is inferior in productivity and a pressure of about several hundred kilograms per square centimeters or higher is generally required as a pressing pressure in order to obtain a material of sufficiently high strength and modulus.